Acute Graft versus Host Disease (aGvHD) is the most deadly complication facing children who have allogeneic hematopoietic stem cell transplant (HSCT). aGvHD occurs, in large part, because the T cells in the bone marrow graft do not "accept" the presence of the transplant recipient's cells, and mount a severe, debilitating, and often deadly attack against the recipient, striking the skin, the liver, and the gastrointestinal track, most prominently. For patients receiving bone marrow from an unrelated donor, the rate of aGvHD can reach as high as 80%, with up to half of patients dying from this complication. These serious outcomes occur despite our best efforts at aGvHD prevention. Given the lack of success in preventing aGvHD with current therapies, novel therapies to prevent this disease are desperately needed.